The present invention relates to scintillation detectors for measuring ionizing radiation at successive depths of bore holes in the earth, and more particularly to an improved scintillation detector window assembly better able to withstand adverse operating conditions than detector window assemblies of the prior art.
U.S. Pat. No. 4,004,151, assigned to the assignee of the present application and incorporated herein in its entirety by reference, discloses a frustoconical glass window set large face first into the machined end of the tubular detector casing and bonded to it by an epoxy adhesive composition that establishes a relatively thick, annular, hermetic seal of varying wall thickness between the window and the machined end of the detector case.
While a detector in accordance with U.S. Pat. No. 4,004,151 represents a substantial improvement over earlier scintillation detectors of the subject type, failure of its window assembly, though infrequent, has occurred under adverse operating conditions including high temperature (150.degree.-200.degree. C.).
It has been determined by investigation that increased heating of the cylindrical scintillation crystal, coaxially mounted within the tubular detector case, as it is lowered into a bore hole causes longitudinal expansion of the crystal, which in turn applies a compressive force to the inner face of the window, in addition to a normal biasing force. If the additional force applied by the expanding crystal is high enough, the epoxy adhesive composition sealing annulus bonding the glass window to the machined end of the detector case may fail, such as by cracking, wherein the hermetic containment of the hygroscopic crystal will be lost, resulting in eventual failure of the detector.
The desire to improve the window assembly of the detector illustrated by U.S. Pat. No. 4,004,151, together with the belief that failure of such a prior art epoxy-bonded window assembly was possibly due to undesirable aspects of the window-seal-casing interface geometry, led to the present invention.